Article

The black poplar (Populus nigra)

Although the black popular can
be found nearly all over Europe, it is one of the rarest and most endangered trees. The main reason is
the eradication of the alluvial areas, which are the natural habitats of this
quick growing giant. But the cultivation of hybrid poplars also contributes to
the decline of this species which is threatened with extinction.

World wide there are around 60 species of poplar. They
belong to the willow family. The quick growing black poplar needs only 30 to 50
years to grow into a majestic tree of around 30 m in height and 2 m in trunk
diameter. Two different forms distinguish this species of tree.

The original black poplarwith a
wide crown

The pillar like Lombardy
poplar(Populus nigra "Italica")

The former is now only to be found occasionally, along
river banks and on flood plains. The latter along roads. In spite of their quick
rate of growth black poplars can reach an age of 100 to 150 years.

Characteristics and uses

The dark grey, almost black bark with its furrowed
structure gave this species of popular its name. Its crown is wide, but irregular,
and the trunk is often gnarled and covered with burls. The unusual trunk form
is characterized by a board like root base.

Fig. 2 - A black poplar (Populus nigra) with its typical burls,
which not all trees have.Photo: Daniela Csencsics (WSL)

The buds are close together, sticky and brownish in
colour. The 8 to 12 cm long leaves which twist around their stems are diamond
shaped, nearly triangular with long drawn out points. The catkins of the black
poplar are around 10 cm long and appear in April growing on separate trees
according to gender (dioecious). Poplars reproduce not only by means of fluffy
seeds which float out of the burst fruit capsules (so called poplar snow) but
also vegetatively through shoots and suckers. To preserve this tree species,
tree nurseries propagate black poplars by means of cuttings or grafting.

The usefulness of black poplar timber is limited not
only because it is bent, knobbly and knotty, but mainly because it is not very
durable. The light wood, which has a dark centre, is soft and easy to work with
and is mainly used for making palettes, chests, and matches and pressed wood.
Wood carvers carve mainly clogs. The use of the downy seeds is also
interesting. They are used as fillings for cushions and covers as well as
insulation material. That is why poplars in North
America are often known as “Cottonwood”.

Ecological importance

Like many willow species the black poplar is a typical
tree species of alluvial forests. These forests provide habitats for a large
number of insects. The black poplars are special favourites of the following insects:
the poplar longhorn; poplar leaf beetle; poplar leaf roller; poplar hawk moth as
well as the poplar lutestring and the poplar twig borer. Many species of birds
use the crown as a nesting place. The fact that the black poplar is able to
remove heavy metals out of the soil is also of ecological importance.

Historical and medicinal facts

Fig. 3 - Koehler's Medicinal Plants
(1887)

According to Greek mythology the poplars emerged from
the Heliades, the sisters of Phaeton, the son of the sun god Helios. As he was
not able to rein in the horses after they bolted with their father’s sun coach
he was hit with a punishing lightning bolt by Zeus. Out of grief over their dead
brother the Heliades turned into black poplars. That is why this species of
tree was associated with death during antiquity.

4000 years ago the Greek Doctor, Galen, recommended a
cream made out of black poplar buds for inflammation. Today the poplar cream is
still used for skin inflammation and haemorrhoids. Tanning agents and ethereal
oils, whose substances are fever reducing and disinfecting, are extracted out
of the buds. Napoleon was responsible for a rapid spread of the Lombardy poplar
in the 19th Century. He had his military roads lined with this quick
growing species of tree, which originated in Italy,
for better orientation in winter and shade in summer.

The black poplar in the laboratory

Native black poplars are not only endangered because
of their declining habitat – the alluvial forests – but also because of a
possible cross breeding with hybrid poplars. Therefore as many pure black
poplars as possible should be cultivated and planted. However, this is not easy
as it is very difficult to ascertain the purity of this species on its
appearance alone.

Scientists of the WSL working together with the ETH on
the project "Promoting rare tree species" (SEBA) tested over 1’000 poplar
samples using genetic methods (DNS Analysis). Within a few weeks using only a
few milligrams of wood it could be seen whether the samples, which had been
collected in different Cantons, were really pure black poplars or hybrids. What
is encouraging is that these typical alluvial forest trees are more wide spread
than once imagined. A prudent estimate of numbers is around 65’000 black
poplars.